Melanomas in Horses

Editorial Information

David Rendle BVSc MVM CertEM(IntMed) DipECEIM MRCVS

Melanomas tend to occur in horses of over 6 years of age in
mares, geldings and stallions alike. They virtually always occur in
grey horses although other colours may on occasion be affected.
Arabs, Percherons and Lipizzaner breeds may be more commonly
affected and certain family lines have also been identified to be
predisposed to the condition probably as a result of in-breeding.
Regardless of breed, most grey horses will develop melonomas with
virtually all greys developing the condition by their late
teens.

Figure 1: A small raised plaque to the right
of the anus indicating early melanoma
development.

Aetiology

Melanomas are tumours of pigment containing cells in the skin
called melanocytes. Their cause is unknown but they are not thought
to be induced by exposure to ultraviolet light as they are in man.
They are generally more benign than human malignant melanomas.

Clinical Signs

The most common site for melanoma development is the underside
of the tail and around the anus. Other sites on the tail may also
be affected with the tumour also seen around the lips, at the base
of the ear around the parotid salivary glands, within the guttural
pouches, around the eyes and on the external genitalia. The tumours
may appear to be within or beneath the skin and may or may not be
covered with hair. They are usually firm and nodular or
plaque-forming. Where they are not covered in hair they are dark
black in colour. As they develop they may become ulcerated or ooze
a thick, black, oily discharge. Nodules often become coalescing as
they grow.

Figure 2: More advanced melanoma formation
around the anus

Figure 3: Melanomas beneath the ear in the
parotid region.

Figure 4: Melanomas beneath the ear in the
parotid region.

Melanomas usually grow slowly over a number of years but they
can at any point start to grow more rapidly and spread to internal
organs. Around two thirds of horses will have evidence of internal
spread at post mortem examination; however, in the vast
majority of cases there are no clinical signs related to the
internal spread.

Figure 5: Nodular melanomas on the
penis

Diagnosis

Diagnosis is usually based on characteristic clinical appearance
in grey horses. However, where there is doubt examination of a
biopsy enables confirmation of the diagnosis.

Treatment

Medical treatment with the oral drug cimetidine was once popular
but has largely been discredited. Cimetidine is expensive and in
the absence of proof that it works is not recommended. In cases
where it was reported to have worked it tended to cause a perceived
slowing of the growth of the tumours rather than remission.

Surgical excision is the mainstay of treatment and is often
accompanied by cryosurgery (freezing). Incomplete excision may
increase the risk of aggressive re-growth or metastasis. Despite
this there is an increasing trend toward aggressive removal of
melanomas when they appear and before they become too large to
effectively excise.

Chemotherapuetic agents such as cisplatin are also effective in
treatment.

The decision whether to treat or monitor early tumours is one of
owner and veterinarian preference.

Welfare Implications

Melanomas rarely cause welfare concerns until they become
extremely large and result in discomfort or prevent normal body
functions. Internal tumours rarely cause any clinical signs. The
most common clinical problem is extensive involvement of the anus
and tail-base that ultimately may prevent normal passage of faeces.
Cases with extensive peri-anal involvement often have to be
euthanased eventually. Thankfully progression is usually slow over
many years. Tumours around the parotid salivary glands may
interfere with tack as they become larger. If tumours become
ulcerated they can become infected and painful.

Disease Control and Prevention

There are no specific steps that can be taken to prevent the
condition. Owners should accept that most grey horses will develop
the condition by their late teens. The unpredictable nature of
tumour growth makes it difficult to give specific recommendations
related to the risks associated with purchasing a horse with
melanomas. The decision on when to treat is also based on opinion
rather than science.

Summary of Key Learning Points

Melanomas are a common tumour of older grey horses

Their progression is unpredictable

They will spread to internal organs but this rarely causes
clinical problems

Surgery with freezing is the mainstay of treatment

The decision on when to treat is difficult. Surgery may
increase the risk of re-growth if it is incomplete but if delayed
it is less likely to be successful

Medical therapy with cimetidine is unlikely to be
effective

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